Can you install apps on the HTC Desire using the computer?

If I don't have access to wifi but also don't want to download apps using the internet allowance on a contract, can I download them through the computer and get them installed on the phone that way? (HTC Desire)

Sure - if you know what apps your looking for, use google and search "[apps name] download .apk" (apk = android installer file) For example => http://www.apkdownload.com/ Be sure though that you're downloading correct version / legal versions.
Oh, and you will need a file explorer such as Astro file manager on your Desire in order to find the .apk and install it.

So: you download the .apk to your PC. Then transfer it via USB to your Desire. Then open Astro (or any file manager); search for your .apk click on it, and Astro will ask if you want to open the file or open the installer. Hit "install". Done.
Astro will also back up all your apps to SD - handy when you get a different Android phone. You can just reinstall all your apps from the SD card rather than download them all over again